Participation criterion
This is the criteria that there is no strategic advantage to not vote. Typically a voter does not know they are in this situation and if they do they can potentially avoid the problem with Tactical voting
Statement of Criterion
Adding one or more ballots that vote X over Y should never change the winner from X to Y.
Variants
Weak Participation criterion
By voting, you cannot cause X to be elected instead of Y (with all other winners staying the same) if you scored Y higher than X. One consequence of this criteria, is that by voting, you can never get a result that is less desirable according to your ballot then a result that is more desirable according to your ballot. The difference between this criterion and the strong Strong Participation criterion is that because unless there was an infinite range for you to score candidates on (you are scoring candidates on infinite sliding bars), due to the approximations you make when expressing your utilities on a finite number of scores, your ballot can disagree with you about whether outcome X is better than outcome Y.
(Described here: "Optimal proportional representation" multiwinner voting systems I: methods, algorithms, advantages, and inherent flaws)
Semi-honest participation criterion
This is a weaker form of the participation criterion. It states that for any set of ballots, an extra voter with a given preference set must be able to cast a ballot which is semi-honest and meaningfully expressive, without making the result worse. Meaningfully expressive means that if the voter prefers some set of candidates to the winner, the non-harmful ballot must be able to express that preference.
Complying Methods
This Criteria is important in the context of the Balinski–Young theorem. Failing the participation criterion is an an example of failing Population monotonicity.
Plurality voting, Approval voting, Cardinal Ratings, Borda count, and Woodall's DAC and DSC methods all pass the Participation Criterion. Condorcet methods, Majority Choice Approval, and IRV fail.
All Monroe type Multi-member systems fail participation.
Some parts of this article are derived with permission from text at http://electionmethods.org
Notes
The Participation criterion offers one way of justifying Score voting being no worse than FPTP: voters can never be hurt by casting an FPTP-style vote (bullet voting). Many other voting methods, such as Condorcet, IRV, and STAR fail even this weaker version of Participation.